LRRK2 and Parkinson's disease cell biology Parkinson's disease is a disorder of movement, cognition and emotion, characterized neuropathologically by neuronal degeneration and deposits of protein aggregates termed Lewy bodies. While most cases are sporadic, rare genetic forms of the disease, caused by mutations in alpha-synuclein, parkin, DJ-1 and PINK1, are helping to elucidate pathogenesis. In previous studies, we have defined the role of alpha- synuclein protein interactions (including interactions with synphilin-1 and parkin) in PD-related cell biology. Mutations in leucine-rich repeat kinase 2 (LRRK2) have recently been found to cause autosomal dominant PD. Our overall hypothesis is that identifications of LRRK2 protein interactions will help elucidate pathogeneses of LRRK2 related PD, and possibly sporadic PD. We have identified interactions between LRRK2 and several other proteins, including parkin, synphilin-1, WSB-1 and CARD7. We have found that that mutant LRRK2 causes direct cellular toxicity, and have initial data that for at least some of the LRRK2 mutations, GTP binding and kinase activity are necessary for toxicity. In Specific Aim 1 we will identify LRRK2 interacting proteins using the yeast two-hybrid system and co-immunopreciptation from transfected cells, and define interaction domains of these proteins. In Specific Aim 2 we will study the LRRK2 interactions in expression studies in cells in culture, and in mouse and human tissue, including postmortem human sporadic and mutant LRRK2 PD tissue. We will study the role of these interactors in LRRK2 cellular toxicity, by using siRNA, and by modifying the interaction domains. In Specific Aim 3 we will determine whether LRRK2 kinase activity is critical for cell toxicity, and determine whether LRRK2 can phosphorylate the interactors-and if so, we will determine whether this has a role in toxicity. These studies will help define the role of LRRK2 and its interacting proteins in cellular pathogenesis related to PD, and potentially identify targets for future therapeutic interventions.